1. Field of the Invention
The present invention is directed to a method and apparatus for converting biomass, coal, or quality fuels into a producer gas, and more particularly, to a latent heat-ballasted gasifier system.
2. Description of the Related Art
Considerable progress has been made in the last few years in developing large gasifier/gas turbine power plants that improve energy efficiency and markedly reduce pollution emissions, compared to direct coal combustion. However, these plants cannot be scaled-down to sizes appropriate to many applications without losing the economic advantages realized at the larger scales. The reasons for this become apparent upon examination of the present approaches to gasification.
Gasification consists of two distinct processes--combustion and pyrolysis. Pyrolysis is the chemical decomposition of solid or solid/liquid fuels through the release of volatile compounds at elevated temperatures. Pyrolysis produces combustible components such as CO, H.sub.2, CH.sub.4, and a variety of condensible organic compounds referred to collectively as tar. Several non-combustible gases are also released, primarily N.sub.2, CO.sub.2, and water vapor.
Since the overall process of pyrolysis is endothermic, a source of high temperature heat is required to drive the reactions. This heat is generally provided by the second process of gasification: combustion. In particular, a small portion of the solid fuel is burned to provide sufficient heat to pyrolyze the remaining fuel.
One approach combines combustion and pyrolysis in the same reaction vessel. Sufficient oxygen is admitted to burn part of the fuel which then heats surrounding unburned fuel to pyrolysis temperatures. Many gasifier configurations have been developed to accomplish this process, but these methods dilute the product gas with incombustible constituents, degrading the heat value of the producer gas. For example, if air is used as the source of oxygen for combustion, almost four volumes of inert nitrogen will appear in the product gas for every one volume of oxygen required for combustion. Air-blown gasifiers contain as much as 45% nitrogen by volume, resulting in heating values as low as 150 Btu/scf (Btu/cubic feet) compared to 1000 Btu/scf for natural gas. Larger piping, greater pumping penalties, larger power machinery, and/or reduced load capacity are some of the penalties associated with the use of such low heating value fuels.
Nitrogen can be eliminated from the product gas by substituting pure oxygen to burn the fuel. Use of pure oxygen can boost the heating value of the producer gas by 85% to about 280 BTU/scf. However, producer gas from oxygen-blown gasifiers still contains as much as 20% diluent gas in the form of CO.sub.2 as a result of combustion. Furthermore, oxygen-blown gasifiers require dedicated air-liquefaction plants to supply the large quantities of oxygen consumed in the process. Such high capital cost facilities are economically justified only for large-scale gasification plants that are well in excess of the capacity appropriate to biomass energy systems.
An alternate approach that avoids use of oxygen in the production of medium BTU gas is known as indirectly heated gasification. In this approach combustion and pyrolysis are physically separated with the result that products of combustion do not appear in the producer gas. Heating values as high as 380 BTU/scf are possible, which is 36% higher than possible with oxygen-blown gasifiers and 230% higher than can be obtained with air-blown gasifiers. Furthermore, no expensive air-liquefaction plant is required to generate this medium Btu gas. Several different schemes of indirect gasification and their shortcomings are described below.
Several alternative schemes for indirect gasification have been suggested. These include transferring hot solids from the combustor to the pyrolyzer, transferring a chemically regenerable heat carrier between these two reactors, transferring heat through a wall common to the reactors, and alternating combustion and pyrolysis in a single reactor. All these methods have proved unsatisfactory as presently practiced.
Transferring hot solids between two reactors is technically feasible but solids handling at temperatures exceeding 1000.degree. K. is fraught with many practical difficulties, including high recycle rates to keep the pyrolysis reactor hot and accelerated abrasion and wear at high temperatures. Use of a regenerable chemical reaction that has a high heat of reaction is an attractive concept, but suffers many of the solids handling problems of the hot solids transport approach. Identification of a suitable chemical energy carrier has also hampered further development of this concept. Heat transfer through a separating wall appears to be an obvious and straightforward solution to the problem but requires extraordinary high convection coefficients to move the large quantity of heat at the necessary temperatures. Overall heat transfer rates have been limited by the need to overcome thermal boundary layers on both sides of the separating wall. There is also concern that a gas leak through the wall could lead to an explosion in the pyrolyzer.
Another approach to indirect heated gasification alternates combustion and pyrolysis in a single reactor. This cyclic process begins with combustion of solid fuel in air to release heat that is stored in some high heat capacity material within the reactor. The combustion phase is followed by a pyrolysis phase in which steam is substituted for the air. The fuel is pyrolyzed to producer gas. The heat for this energy absorbing process comes from the heat stored in the reactor during the combustion phase.
Alternating combustion and pyrolysis in a single reactor to produce "water gas," is the only indirectly heated gasification process which has met with commercial success. At the turn of the century, water gas was produced by heating coal in an air stream until it glowed red. At this point, air was shut off and steam injected into the reactor. The endothermic reaction of steam and coal char was supported by the sensible heat capacity of the hot char. The reactor temperature and the rate of gasification quickly dropped as the stored heat was consumed by the endothermic reaction. The cycle was repeated by shutting off the steam and readmitting air. By today's standards, these water-gas reactors are inefficient, polluting, and uneconomical.